At the start of a new year, lots of people will resolve to make a healthy lifestyle change.
Many find resolutions like cutting back on unhealthy snacks or taking part in a weekend fitness class easier when friends and family are making the same changes.
However, not all decisions affecting our health are intentional, as we copy the behaviour of friends, colleagues and family who we relate to and admire.
Unfortunately, we also imitate habits that are bad for our health, like smoking or eating too much.
This phenomenon means non-contagious conditions like heart disease, strokes and cancer can appear to spread from person to person like an infection.
Can your friends make you obese?
People whom we value and are in regular contact with, form our social network.
The Framingham Heart Study has studied the power of social networks since the late 1940s, by tracking three generations of residents in Framingham, a Massachusetts city.
The research indicated a person was far more likely to become obese if someone in their circle had also become obese. It suggested they were 57% more likely if it was a friend, 40% if it was a sibling, and 37% if it was their spouse.
The effect was more pronounced if the two people were of the same gender, and was linked to how strongly the individual felt about the other person.
For example, the Framingham study indicated a person’s weight was not affected by that of a neighbour they saw daily if they didn’t have a close relationship.
In unbalanced friendships, the person who saw the friendship as important was more likely to put on weight if their “friend” did, but not the other way around.
The level of divorce, smoking and alcohol drinking also appeared to spread via friends and family.
These findings are important. Although we are affected by ageing and can be predisposed to certain conditions, our risk of developing the most common non-infectious diseases is significantly increased by certain behaviours:
- Whether you smoke
- Your diet
- How much physical activity you do
- How much alcohol you drink
These non-infectious conditions – including heart disease, stroke, cancer, diabetes and lung disease – cause seven out of every 10 deaths globally and nearly 90% of all deaths in the UK.
Emotions are catching
Social networks may also affect our behaviour and mood.
Perhaps unsurprisingly, smoking in teenagers may be influenced by popularity. When popular adolescents smoke, overall levels of smoking increase and the number of people who quit falls.
Separately, young people whose friends suffered from low mood were found to be more likely to develop low mood themselves and vice versa.
These symptoms didn’t amount to clinical depression, which was not found to spread. But low mood is known to affect teenagers’ quality of life and can sometimes lead to greater risk of clinical depression later on.
The idea that emotions are catching is backed up by a controversial experiment secretly conducted on almost 700,000 Facebook users.
The experiment selectively filtered what could be seen on users’ news feeds, which use an algorithm to show relevant posts from their Facebook friends.
Two parallel experiments were conducted; one reduced users’ exposure to posts displaying positive emotion, while the other reduced exposure to posts featuring negative emotion.
Users who encountered positive posts were more likely to post positively themselves, and vice versa. This suggests emotions may spread through online social networks, despite a lack of face-to-face interaction or body language cues.
One criticism levelled at studies of our social networks is that we become friends with people who already have similar traits to us or are in a similar situation. But many studies try to account for this theory, known as social contagion.